Ultrasonic Imaging In Wells Or Tubulars

ABSTRACT

An ultrasonic imaging method is provided. A wideband acoustic pulse is fired at a wall. A wideband response signal is received. The wideband response signal is processed to select an impedance measurement frequency. A wavelet having a characteristic frequency approximately equal to the impedance measurement frequency is fired. A wavelet response signal is received. A reflection coefficient is determined from the wavelet response signal. An impedance measurement is calculated from the reflection coefficient. Related tools and systems are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application includes subject matter related to the followingapplications, which are hereby incorporated by reference: U.S.Provisional Patent Application No. 60/707,717, dated Jul. 22, 2005,entitled “Ultrasonic Imaging in Wells or Tubulars,” by Mandal et al.;U.S. Pat. No. 6,041,861, dated Mar. 28, 2000, entitled “Method todetermine self-calibrated circumferential cased bond impedance,” byMandal et al.; U.S. Pat. No. 6,661,737, dated Dec. 9, 2003, entitled“Acoustic Logging Tool Having Programmable Source Waveforms,” byWisniewski, et al.

BACKGROUND

It is often desirable to determine the thickness of a pipe in a well andthe properties of materials that surround the pipe, or the well. Forexample, it may be desirable to determine whether the exterior of a pipeis in contact with fluids or solids, and if so, what type of fluids orsolids. Thinning of a pipe due to corrosion may indicate a potentialcollapse of the pipe. In a downhole context, it may be desirable todetermine whether the pipe is tightly bonded to surrounding cement.These are just some examples of when it may be desirable to measure (oreven image) through a pipe, or solid tubing wall.

One method for performing such measurements employs an acoustictransducer. Unfortunately, existing technology is limited to pipeshaving walls less than 0.9 inches thick.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description, wherein like reference numerals represent likeparts.

FIG. 1 shows a pipe-conveyed logging environment.

FIG. 1A shows an acoustic scanning tool embodiment.

FIG. 2 shows an acoustic transducer transmitting an acoustic pulsethrough a fluid-casing-cement configuration.

FIG. 3 shows illustrative acoustic response component waves received byan acoustic transceiver.

FIG. 4 shows an illustrative frequency response of thefluid-steel-cement configuration for different frequencies andthicknesses.

FIG. 5 shows an ultrasonic imaging method for wells or tubularsaccording to an embodiment of the present disclosure.

FIG. 6 shows an illustrative casing-evaluation presentation.

DETAILED DESCRIPTION

It should be understood at the outset that although implementations ofvarious embodiments of the present disclosure is described below, thepresent system may be implemented using any number of techniques,whether currently known or in existence. The present disclosure shouldin no way be limited to the implementations, drawings, and techniquesdescribed below, but may be modified within the scope of the appendedclaims along with their full scope of equivalents.

The present disclosure provides an ultrasonic imaging method that may beused for pipes having walls greater than 0.9 inches thick. In someembodiments, an acoustic transducer generates a wideband acoustic pulseto determine an impedance measurement frequency for a pipe wall, andthen generates a wavelet having a characteristic frequency approximatelyequal to the impedance measurement frequency for the pipe wall. Aprocessor determines a casing thickness from the inter-peak time in thewavelet response signal. The processor also determines a reflectioncoefficient from the wavelet response signal. Then the processorcalculates an impedance measurement from the reflection coefficient,where the impedance measurement indicates whether the exterior of thepipe is in contact with fluids or solids, and if so, what type of fluidsor solids, even if the pipe has walls greater than 0.9 inches thick. Theprocessor may further associate position and orientation informationwith the impedance measurements to generate an image.

Turning now to the figures, FIG. 1 illustrates a typical pipe-conveyedlogging environment. Continuous tubing 14 is pulled from a spool 12 andinjected into a wellbore by an injector 16. The tubing 14 is injectedthrough a packer 18 and a blowout preventer 20, and passes through acasing 22 into the wellbore. In the well, a downhole instrument 24 iscoupled to tubing 14. The downhole instrument 24 may be configured tocommunicate to a surface computer system 26 via information conduitscontained in the tubing 14. A power supply 28 may be provided to supplypower to the downhole instrument 24 via power conduits in the tubing 14.Alternatively, the power and telemetry may be conveyed by a wirelineattached to the surface of the tubing 14.

The surface computer system 26 may be configured to communicate with thedownhole instrument 24. The downhole instrument 24 may include severaldownhole sensors and control devices. The surface computer system 26 maybe configured by system control software 30 to monitor and controlinstruments in the downhole instrument 24. The system 26 may include auser-output device 32 and a user-input device 34 to allow a humanoperator to interact with the software 30. The surface computer system26 may provide programmable acquisition parameters, includingprogrammable record time for receive signals, programmable samplingintervals, and programmable spatial coverage (resolution).

FIG. 1A shows one embodiment of an acoustic scanning tool 100. Theacoustic scanning tool 100 may be one of the downhole sensors includedin the downhole instrument 24. The acoustic scanning tool 100 may bedivided into sections, including: a main electronics section 102, adirectional sub section 104, an acoustic transducer 200, and a secondacoustic transducer 106. The acoustic scanning tool 100 has aninterchangeable head that may rotate a full 360 degrees and contains theacoustic transducer 200 to provide a full 360 degree profile of thecasing 22 or the borehole surrounding the casing 22. A turning motor mayrotate the acoustic scanning tool 100.

The main electronics section 102 controls the acquisition of thewaveform data by the acoustic transducer 200 and communication with thesurface computer system 26. The signals from the acoustic transducer 200may be digitized using high resolution (e.g., 16 bit) analog-to-digitalconverters (ADC) and transmitted to the surface computer system 26. Thedirectional sub section 104 provides orientation measurements thatenable the surface computer system 26 to generate images frommeasurements by the acoustic transducer 200. In some logging conditions,the images produced may consist of 200 points horizontally by 40 samplesper foot vertically. The second acoustic transducer 106 may be mountedin the acoustic scanning tool 100 to measure characteristics of anyborehole fluid to produce real-time fluid travel time (FTT)measurements, as an alternative to using FTT estimates supplied by thesurface computer system 26. The second acoustic transducer 106 may alsomeasure borehole fluid impendence to correct measured data.

The data acquisition speed for the surface computer system 26 may belimited by the net telemetry rate, the desired depth sampling rate, therate for rotating the acoustic transducer 200, the rate for pulling theacoustic transducer through the well, and/or the processing power of thesurface computer system 26. In one embodiment, the acoustic transducer200 may employ piezoelectric elements designed to operate in downholeconditions. However, other transducers that are suitable for downholeoperation may be used. The acoustic transducer 200 may be configured toswitch back and forth between transmit and receive modes.

The main electronics section 102 fires the acoustic transducer 200periodically, thereby producing acoustic pressure waves that propagatethrough the borehole fluid and into the casing 22 and the surroundingformation. At the inner casing wall, some of the acoustic energy isconverted into waves that travel through the formation, and some of theacoustic energy is converted into reverberations that propagate back tothe acoustic scanning tool 100. As these reflections propagate back tothe acoustic scanning tool 100, they cause pressure variations that maybe detected by the acoustic transducer 200. The signals received by theacoustic transducer 200 may be processed by the surface computer system26 to determine the characteristics of the casing 22 and the formationsurrounding the casing 22.

FIG. 2 illustrates an acoustic wave path with casing reflections for adownhole acoustic wave. Shown are the acoustic transducer 200, adrilling fluid 210, a steel casing 220, a cement bond 230, and a portionof a surrounding formation 240. Although depicted as steel casing 220,the casing 22 may be composed of materials other than steel, may be inthe form of a pipe, or tubular, and may be referred to as a wall or acasing wall. The drilling fluid 210 (which could alternatively beproduction fluid) acts as a transmission medium while occupying the areabetween the acoustic transducer 200 and the steel casing 220, an areareferred to as the annulus. The drilling fluid 210, the steel casing220, the cement bond 230, and the surrounding formation 240 each have acorresponding impedance, labeled Z_(m), Z_(x), Z_(c), and Z_(f),respectively. Also shown is an acoustic pulse 250, including a firstreflected portion 260, casing wave portions 270, 271, 272, 273, 274, 275and cement wave portions 280, 281, 282. Reflections of cement waveportions 280-282 also propagate through the cement, although this is notexplicitly shown.

To measure the impedance of cement or other material outside of thesteel casing 220, the acoustic transducer 200 acts as a transmitter tosend out the acoustic pulse 250, with a characteristic frequency in therange between 195 to 650 kHz, (or in some implementations, 100 kHz to750 kHz) then switches to the receive mode. The acoustic pulse 250frequency is focused on an expected resonance frequency of the steelcasing 220. The acoustic pulse 250 travels through the drilling fluid210 and strikes the steel casing 220. The largest portion of the energyof the acoustic pulse 250 is reflected back to the acoustic transducer200 as the reflected portion 260 while a small amount of signal entersthe steel casing 220 as the casing wave portion 270. When the drillingfluid 210 is water, the reflected portion 260 has an expected amplitudeof about 93% of the acoustic pulse 250.

The portion of the acoustic pulse 250 that enters the steel casing 220is reflected back and forth between the steel casing 220/drilling fluid210 interface and the steel casing 220/cement bond 230 interface, asillustrated by the casing wave portions 271-275. At each reflection someenergy is transmitted through the interface, dependent on the acousticimpedance contrast, and is either directed back toward the acoustictransducer 200 or out to the cement bond 230. The casing wave portions271-275 inside the steel casing 220 are quickly dissipated in thismanner at a rate directly dependent on the acoustic impedance of thematerial outside the steel casing 220 according to the equation:R ₁=(Z ₁ −Z ₂)/(Z ₁₊ Z ₂)  (1)where R₁=the reflective coefficient, and Z₁ and Z₂ are the impedances ofthe materials at the interface in question.

The acoustic transducer 200, now acting as a receiver, detects anacoustic response, a waveform consisting of a sequence of exponentiallydecaying peaks, beginning with the loud initial reflection. The peak topeak times equal twice the travel time through the steel casing 220.

A distinguishing feature between the illustrative acoustic responsewaveforms shown in FIG. 3 is the casing thickness relative to thecharacteristic wavelength of the acoustic pulse 250 from the acoustictransceiver 200. For a casing thickness that is on the order of a ½wavelength of the acoustic transceiver 200 frequency, the acousticresponse waveform may be expected to be an exponentially-decayingoscillatory signal. For a casing thickness that is much higher than the½ wavelength (also assuming the acoustic transducer 200 ring-down timeis less than the two-way travel time of the casing wave), the waveformmay be expected to be a pulse train with exponentially-decayingamplitudes. The decay rate of such a pulse train may be relativelystraightforward to measure, as it is calculable as the amplitude ratiobetween neighboring pulses (not including a first pulse amplitude 302),such as the ratio between a second pulse amplitude 304 and a third pulseamplitude 306. This decay rate is directly proportional to thereflectivity coefficient given in equation (1), so with a known acousticimpedance for the steel casing 220, the impedance is readily determinedfor the cement bond 230. If the impedance determined for a portion ofthe cement bond 230 is lower then the determined impedances for thesurrounding portions of the cement bond 230, the lower impedance mayindicate that the cement bond 230 has corroded at the portion with thelower impedance.

The acoustic transducer 200 may be fired repeatedly as the acoustictransducer 200 rotates and moves along the steel casing 220, enablingmeasurements to be taken over many points on the steel casing 220. Suchmeasurements may be used to create a log and/or image of the steelcasing 220 and the material on the outside of the steel casing 220.

The surface computer system 26 uses the relationship between thewavelength and the frequency of a wave when selecting the frequency ofthe wave generated or transmitted. The wavelength of a wave is thedistance between repeating units of a wave. Because the frequency of awave is the number of peaks of a wave to pass a point in a given time,the wavelength of a wave has an inverse relationship to the frequency ofa wave. Therefore, when the surface computer system 26 doubles thefrequency for a wave, the wavelength of the wave is divided in half.

It has been discovered that fundamental reverberation mode measurementsof thick-walled casings would require high power, low frequencytransducers with a long listening time. Many transducers do not functionas accurately at low frequencies, and long listening times produce moresignal noise. Ultrasonic measuring systems that operate at thefundamental reverberation mode (i.e. the frequency at which thewavelength is twice the casing thickness) are feasible only for casingshaving thicknesses below about 0.8 inches. For example, a frequency of236 kHz may produce the fundamental reverberation mode for a casing thatis 0.474 inches thick, but a casing that is 0.948 inches thick mayrequire a frequency of 118 kHz, which is a frequency that is so low thatthe signal strength for the reverberation portion of the wave becomes aproblem. Additionally, the requirement for a long listening time resultsin slower data acquisition speed and more noise.

Instead of decreasing the frequency of a wave to operate at thefundamental reverberation mode for a thick casing, embodiments of thepresent disclosure increase the frequency of a wave to operate at a highorder reverberation mode, which is an non-unitary integer multiple ofthe fundamental reverberation mode. For example, rather than operatingat the fundamental reverberation mode by selecting a wavelength of 1.896inches for a casing that is 0.948 inches thick, an embodiment of thepresent disclosure may select a wavelength of 0.948 inches, one half ofthe wavelength of 1.896 inches required for the fundamentalreverberation mode. Because the wavelength is reduced through divisionby two, the frequency is increased though multiplication by two.Multiplying the low frequency of 118 kHz required for the fundamentalreverberation mode by two results in a frequency of 236 kHz, a mid-rangefrequency that does not share the signal strength problems associatedwith the low frequencies around 118 kHz. Because the wavelength of thewave equals the thickness of the casing, when the reflected portion ofthe wave travels twice the distance of the thickness of the casing, thereflected portion of the wave completes two full periods of thewavelength. Completing two full periods creates constructiveinterference, which produces a stronger receive signal at the acousticscanning tool 100. If the frequency required for the fundamentalreverberation mode is multiplied by three, the reflected portion of thewave completes three full periods in each round trip through the casing.

Noise in the signal for a pulse train with exponentially-decayingamplitudes may be filtered out by using a theoretical response for thepulse train. The theoretical prediction of the reflection waves isobtained by multiplying the frequency domain source signal, S(ω), withthe frequency domain theoretical response, R(ω). This is equivalent to aconvolution in the time domain of the acoustic transducer 200 waveletwith the impulse response of the impedance profile shown in FIG. 2.Assuming a flat casing, the theoretical frequency domain response ofnormal incidence wave may be modeled by the following expression:$\begin{matrix}{{R(\omega)} = {\frac{Z_{m} - Z_{s}}{Z_{m} + Z_{s}} + {\frac{\frac{4Z_{m}Z_{s}}{\left( {Z_{m} + Z_{s}} \right)^{2}} \cdot \frac{Z_{s} - Z_{c}}{Z_{s} + Z_{c}}}{\left( {1 - {\frac{Z_{s} - Z_{m}}{Z_{m} + Z_{s}} \cdot \frac{Z_{s} - Z_{c}}{Z_{s} + Z_{c}} \cdot {\mathbb{e}}^{{- {\mathbb{i}}}\quad 2\omega\frac{C_{t}}{V_{s}}}}} \right)} \cdot {\mathbb{e}}^{{- {\mathbb{i}}}\quad 2\omega\frac{C_{t}}{V_{s}}}}}} & (2)\end{matrix}$whereR(ω)=the reflection coefficient for any angular frequency ω, Z_(m),Z_(s), Z_(c)=impedances for the drilling fluid 210, the steel casing220, and the material on the outside of the steel casing 220,respectively, and V_(s)=the speed of sound in the steel casing 220.

In this expression, V_(s) is the velocity of sound wave in the steelcasing 220 and C_(t) is the steel casing 220 thickness. In equation (2),e^(-i2ωC/V) represents the phase delay of two way travel time inside thesteel casing 220, and the algebraic expressions of Z's are thereflection coefficients along the boundary. For large casing thickness(longer delay) and a high frequency acoustic transducer (smaller widthwavelets, producing waveforms like first response wavelet 402 and secondresponse wavelet 404), the expression (2) may be simplified to representjust the first two pulses in the pulse train: $\begin{matrix}{{R(\omega)} = {\frac{Z_{m} - Z_{s}}{Z_{m} + Z_{s}} + {\frac{4Z_{m}Z_{s}}{\left( {Z_{m} + Z_{s}} \right)^{2}} \cdot \frac{Z_{s} - Z_{c}}{Z_{s} + Z_{c}} \cdot {\mathbb{e}}^{{- {\mathbb{i}}}\quad 2\quad\omega\frac{C_{t}}{V_{s}}}}}} & (3)\end{matrix}$

The 2^(nd) term in equation (3) is proportional to the reflectioncoefficient between the steel casing 220 and the material outside of thesteel casing 220. Thus, the second pulse may be measured as a robustindicator of the reflection coefficient, which in turn enables adetermination of the impedance for the material on the outside of thesteel casing 220 when the steel casing 220 impedance is known orassumed.

In some embodiments, then, the ultrasonic logging tool is provided witha high frequency acoustic transducer having a narrow bandwidth. Theperiod of the source wavelet produced from this high frequency acoustictransducer is less than an expected two-way travel time of sound in thecasing, causing the acoustic response waveform to take the form of apulse train. The casing thickness may be calculated from the timedifference between two pulses in the pulse train, where the first pulserepresents a reflection from the steel casing 220/drilling fluid 210interface and the second pulse represents the reflection from the steelcasing 220/cement bond 230 interface. A reflection coefficient and/orimpedance for the material outside the steel casing 220 is calculatedfrom the amplitude of the second pulse in the receive waveform incomparison to the amplitude of the first pulse.

In some embodiments of the present disclosure, ultrasonic measuringsystems use higher order reverberation modes. Expression (2) revealshigher-order reverberation modes that propagate in the steel casing 220.These reverberation modes exist independently of the signal pulse width,and do not necessarily require pulse separation in the acoustic responsesignal. FIG. 4 is an illustrative color plot of equation (2) as afunction of casing thickness and acoustic wavelet frequency. Lightercolors indicate higher amplitudes of R(ω), and thereby identify afundamental reverberation mode and various high-order reverberationmodes. For example, a 0.5 inch casing may have dominant signals near 240kHz, or 0.240 MHz, (the fundamental mode) and near other higher ordermodes (480 kHz, 720 kHz) as illustrated in FIG. 4. Similarly, a 240 kHztransducer may excite 0.5 and 1 inch casing. Note that a 480 kHz (˜0.5MHz) acoustic transceiver may obtain measurable responses from 0.25,0.5, 0.75, 1.0, and 1.25 inch casing. That is, a carefully chosenacoustic transducer frequency enables a given logging tool to performultrasonic imaging for casings of many different standard thicknesses.

Some embodiments of the present disclosure provide for excitation ofhigher-order reverberation modes, thereby enabling impedancemeasurements through thick casings without requiring a high-power,low-frequency acoustic transducer. Such embodiments may advantageouslyenable one acoustic transducer frequency to be used for measurementswith multiple casing wall thicknesses. In some embodiments of thepresent disclosure, the logging tool may employ a programmable waveformsource such as that described in U.S. Pat. No. 6,661,737, to generatewideband signals, e.g., chirp signals, bandpass white-noise pulses, or“impulse” signals. These wideband signals may be used to identify thecasing reverberation modes, which may then be used to select anarrowband pulse frequency for impedance measurements. The selectedfrequency may correspond to an identified reverberation mode or somenon-unitary integer multiple thereof.

FIG. 5 shows an ultrasonic imaging method for wells or tubularsaccording to an embodiment of the present disclosure. In box 502, theacoustic transducer 200 fires a wideband acoustic pulse at a wall. Thewideband acoustic pulse may be a chirp signal. The wall may be thickerthan 0.9 inches when using embodiments of the present disclosure.

In box 504, the acoustic transducer 200 receives a wideband acousticresponse signal. The acoustic transducer 200 may digitize the widebandacoustic response signal to send a representation of the widebandacoustic response signal to a processor.

In box 506, the processor processes the wideband acoustic responsesignal to select an impedance measurement frequency. The processor mayuse a Fourier transform of the wideband response signal to identifypeaks in the frequency domain. The peaks may represent high-orderreverberation modes. A wavelet is a representation of a signal that isscaled and translated to match the input signal for a decayingoscillating waveform. Based on an identified peak in the frequencydomain, the processor selects an impedance measurement frequency for awavelet.

In box 508, the acoustic transducer 200 fires a wavelet having acharacteristic frequency approximately equal to the impedancemeasurement frequency. The wavelet may have an effective time width lessthan a two-way travel time of sound in the wall.

In box 510, the acoustic transducer 200 receives a wavelet responsesignal. The wavelet response signal may be a pulse train. Theinter-pulse spacing may be indicative of the wall thickness. Theacoustic transducer 200 may digitize the wavelet response signal to senda representation of the wavelet response signal to the processor.Additionally, the acoustic transducer 200 determines the transit time,the time for the travel of the wavelet to the steel casing 220 wall andits time to return back to the acoustic transducer 200.

As such, the transit time provides an indication of the downholedistance between the acoustic transducer 200 and the steel casing 200wall. Multiple measurements of the transit time enable the processor todetermine borehole geometry (ovality), eccentricity and boreholedeviation. The processor may combine multiple measurements of theinter-pulse spacing, indicative of the wall thickness, with the transittime, indicative of the downhole distance between the acoustictransducer 200 and the steel casing 220 wall to produce cross-sectionalsof the pipe shape and the pipe wall. The processor may also use thesecombined multiple measurements to determine the average, maximum, andminimum values of the pipe radius and thickness.

In box 512, the processor determines a reflection coefficient from thewavelet response signal. The processor may determine a peak amplituderatio and a decay rate from the wavelet response signal to determine thereflection coefficient. The processor may convolve a model wavelet witha model response to obtain a model receive signal, compare the modelreceive signal to the received wavelet response signal, and adjust atleast one parameter of the model response to reduce a difference betweenthe model receive signal and the wavelet response signal. The parametersmay include a parameter for thickness and a parameter for impedance. Theprocessor may perform impedance inversion by adjusting parameters of amodel until a model response matches the acoustic responses. The secondpulse in the wavelet response pulse train may have an amplitudeindicative of a cement impedance. Because the 2^(nd) term in equation(3) is proportional to the reflection coefficient between the steelcasing 220 and the material outside of the steel casing 220, the secondpulse may be measured as a robust indicator of the reflectioncoefficient.

In box 514, the processor calculates the impedance measurement from thereflection coefficient. The reflection coefficient enables adetermination of the impedance for the material on the outside of thesteel casing 220, such as cement, when the steel casing 220 impedance isknown or assumed by using equation (1).

In box 516, the processor calculates impedance measurements at multiplepoints on the wall. The processor may calculate impedance measurementsat multiple points on the wall by repeating the method from box 502 tobox 514 for each point on the wall.

In box 518, the processor forms a visual image indicative of theimpedance measurements. The processor may form a visual image indicativeof the impedance measurements by accumulating the calculated impedancemeasurements and wall thicknesses at multiple points on the wall.

FIG. 6 shows an illustrative casing-evaluation presentation. FIG. 6includes visual images that may be formed by executing the method inFIG. 6, specifically box 518. The data in FIG. 6 may be depicted forcasing walls more than 0.9 inches thick by using an embodiment of thepresent disclosure. Track 602 depicts casing ovality, eccentricity, andhole deviation. In this example, the eccentricity is composed of bothtool and casing eccentricity due to formation movement. Track 604depicts a cross-sectional presentation of the pipe shape. Track 606depicts a cross-sectional of the pipe wall. Track 608 provides theaverage, minimum, and maximum value of the pipe radius that is shown intrack 610. Track 612 provides the average, minimum and maximum value ofthe pipe thickness that is the image plotted in track 614. On the imagelogs, red may show pipe thinning while blue may show pipe thickening.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein, but may be modified withinthe scope of the appended claims along with their full scope ofequivalents. For example, the various elements or components may becombined or integrated in another system or certain features may beomitted, or not implemented.

Also, techniques, systems, subsystems and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as directly coupled or communicating witheach other may be coupled through some interface or device, such thatthe items may no longer be considered directly coupled to each other butmay still be indirectly coupled and in communication, whetherelectrically, mechanically, or otherwise with one another. Otherexamples of changes, substitutions, and alterations are ascertainable byone skilled in the art and could be made without departing from thespirit and scope disclosed herein.

1. An ultrasonic imaging method that comprises: firing a widebandacoustic pulse at a wall; receiving a wideband acoustic response signal;processing the wideband acoustic response signal to select an impedancemeasurement frequency; firing a wavelet having a characteristicfrequency approximately equal to the impedance measurement frequency;receiving a wavelet response signal; determining a reflectioncoefficient from the wavelet response signal; and calculating animpedance measurement from the reflection coefficient.
 2. The method ofclaim 1, wherein the wavelet has an effective time width less than atwo-way travel time of sound in the wall.
 3. The method of claim 1,wherein the wavelet response signal is a pulse train, with a secondpulse having an amplitude indicative of a cement impedance.
 4. Themethod of claim 1, wherein the wavelet response signal is a pulse train,with an inter-pulse spacing indicative of a wall thickness.
 5. Themethod of claim 1, wherein the wall is thicker than 0.9 inches.
 6. Themethod of claim 1, wherein the determining comprises measuring a peakamplitude ratio.
 7. The method of claim 1, wherein the determiningcomprises determining a decay rate.
 8. The method of claim 1, whereinthe determining comprises: convolving a model wavelet with a modelresponse to obtain a model receive signal; comparing the model receivesignal to the received wavelet response signal; and adjusting at leastone parameter of the model response to reduce a difference between themodel receive signal and the wavelet response signal.
 9. The method ofclaim 1, wherein the wideband acoustic pulse is a chirp signal.
 10. Themethod of claim 1, wherein the processing comprises performing a Fouriertransform of the wideband response signal and identifying peaks in thefrequency domain.
 11. The method of claim 1, further comprising:calculating impedance measurements at multiple points on the wall; andforming a visual image indicative of the impedance measurements.
 12. Anultrasonic logging tool comprising: an acoustic transducer thattransmits a programmable pulse and receives an acoustic response; and aprocessor coupled to the acoustic transducer to capture the acousticresponse and to determine an inter-peak time and an amplitude of asecond peak in the acoustic response, wherein the processor furtherdetermines an impedance value from the amplitude, and wherein theprocessor still further determines a casing thickness from theinter-peak time.
 13. The tool of claim 12, wherein the processordetermines a characteristic frequency from the casing thickness andprograms the pulse to have an integer multiple of the characteristicfrequency.
 14. The tool of claim 12, further comprising: a secondacoustic transducer to measure characteristics of borehole fluid toproduce real-time fluid travel time measurements.
 15. An acousticlogging system comprising: an acoustic logging tool having an acoustictransducer that transmits acoustic pulses and digitizes acousticresponses; and a surface facility that receives representations of theacoustic responses and determines acoustic impedance measurements ofmaterial around a casing when the acoustic pulses have a characteristicfrequency that is some non-unitary integer multiple of a fundamentalreverberation frequency of a casing wall.
 16. The system of claim 15,wherein the surface facility performs impedance inversion by adjustingparameters of a model until a model response matches the acousticresponses.
 17. The system of claim 15, wherein the characteristicfrequency is designed to excite different reverberation modes indifferent standard casing wall thicknesses.
 18. The system of claim 15,wherein the surface facility determines the impedance measurementsthrough a casing wall thicker than about 0.9 inches.
 19. The system ofclaim 15, wherein the logging tool has programmable acquisitionparameters including programmable record time for receive signals,programmable sampling interval, and programmable spatial coverage(resolution).
 20. The tool of claim 15, further comprising: a secondacoustic transducer to measure characteristics of borehole fluid toproduce real-time fluid travel time measurements.